The present invention relates to an apparatus and method for manufacturing a semiconductor device by using a plasma.
At present, plasma processing technology using a plasma produced in a reactor to perform a variety of semiconductor fabrication processes, including dry etching and CVD, is essential to the processing of components contained in a highly integrated semiconductor device. Before a contact hole for providing a connection between a wire and an active region, such as the source/drain regions of a transistor, or a via hole for providing a connection between wires is formed in an interlayer insulating film composed of an oxide film, a plasma is generated in a reactor such that vertical etching proceeds at the bottom of the hole with ions impinging thereon, while lateral etching is suppressed with a sidewall protecting film covering the sidewall of the hole. Such anisotropic etching allows the formation of a contact hole or via hole having an extremely small diameter and yet extending through the interlayer insulating film substantially perpendicularly thereto.
With the increasing scaling down of a semiconductor device, the width of a wire and the diameter of the contact hole or via hole are also reduced increasingly. However, the interlayer insulating film has not substantially been reduced in size irrespective of the holes reduced in diameter. As a result, the ratio of the thickness of the interlayer insulating film to the diameter of the hole (aspect ratio) has been increased accordingly. With the aspect ratio increasing, there are more cases where normal holes are not formed in the process of forming the holes. In the case of forming a hole with a high aspect ratio, the phenomenon of so-called "microloading" is especially likely to occur in which the etch rate varies depending on the diameter of the hole. If the microloading effect occurs during the formation of a hole having an extremely small diameter, in particular, etching proceeds at progressively lower rates and, in some cases, etching may stop before a through hole is formed.
With recent improvements in dry-etching technology, there have been developed a technique termed TM (Time Modulation) etching whereby the formation of the sidewall protecting film and the etching process are alternately performed in order to maximally suppress the formation of the sidewall protecting film and a technique termed low-temperature etching whereby a wafer is cooled to a temperature at which the reaction of radicals is frozen at the sidewall in order to perform anisotropic etching without forming the sidewall protecting film. These techniques achieve anisotropy, while completely or maximally suppressing the formation of the sidewall protecting film. The techniques also suppress the occurrence of the aforesaid microloading phenomenon.
On the other hand, a low-pressure and high-density plasma such as an inductively coupled plasma or a helicon wave excited plasma has recently been used as a plasma source suitable for etching performed to form a hole with a high aspect ratio. Such a plasma is generated in a reactor with an antenna disposed on a wall portion thereof composed of an insulator. The use of such a plasma achieves high anisotropy, a high etch rate, and a high selectivity to a silicon substrate.
However, when the technique using the high-density plasma is applied to etching, the phenomenon occurs in which etching stops during processing and a through hole is not formed. Such "etch stop" is a phenomenon caused by an abrupt transition in etch rate from a substantially constant value maintained during normal etching to zero. The etch stop does not occur at a fixed time point but occurs at different time points during etching. For example, the etch stop may occur immediately after or some time after the initiation of etching. After several wafers were processed, the etch stop abruptly occurs during the processing of the subsequent wafer, which suggests the presence of some factors conspiring to cause the etch stop.
Although the cause of the etch stop occurring at different time points has not completely been tracked down thus far, the etch stop is a phenomenon obviously distinct from the microloading phenomenon. Hereinafter, the present inventors will refer to such a phenomenon in which etching stops as "etch stop".
FIG. 37 shows etching proceeding in the three cases of normal etching, microloading, and etch stop. In the drawing, the horizontal axis represents an etching period and the vertical axis represents the depth of an etched portion. The etching lines Etn, Eml, and Est represent the respective cases where normal etching is performed, where the microloading occurs, and where the etch stop occurs. As indicated by the etching line Eml shown in FIG. 37, the microloading is characterized by the etch rate lowering when etching proceeds to a certain depth. By contrast, the etch stop is characterized by the abrupt stop of the normal etching process, as indicated by the etching line shown Est in FIG. 37. In addition, the etch stop occurs at different time points during etching as stated previously. In contrast to the microloading effect which is a phenomenon caused by the progressive lowering of an etch rate, the phenomenon of "etch stop" is a phenomenon caused by an abrupt transition in etch rate from a substantially constant value maintained during normal etching to zero. Besides, the etch stop occurs under conditions free from the microloading effect.
In general, the etch rate R during plasma etching is represented by the following equation (1): EQU R=(1/.rho.).multidot..theta..GAMMA.i.multidot.Ei-.theta..multidot..GAMMA.d( 1)
where .rho. is the density of a workpiece to be etched, .theta. is the surface coverage of radicals, .GAMMA.i is a function representing the quantity of ion fluxes, Ei is ion energy, and .GAMMA.d is a function representing the quantity of radical fluxes. The first term of the equation (1) is a factor contributing to etching, while the second term thereof is a factor contributing to deposition.
When etching stops, the etch rate R in the foregoing equation (1) becomes 0 or less. To satisfy R.ltoreq.0, the following conditions should be satisfied.
(a) When .GAMMA.i.apprxeq.0 PA1 (b) When Ei.apprxeq.0 PA1 (c) when (1/.rho.).multidot..theta..multidot..GAMMA.i.multidot.Ei.ltoreq..theta..mu ltidot..GAMMA.d
In this case, the quantity of ion fluxes coming into the contact hole is reduced, which is caused by an increase in the aspect ratio of the contact hole or surface charging.
In this case, the energy of ions coming into the contact hole is reduced, which is particularly caused by the presence of charges inhibiting the ions from moving downward in the contact hole formed in the insulating film. For instance, this is the case where positive charges are present at the bottom of the contact hole and negative charges are present at the upper portion of the sidewall of the contact hole.
In this case, the quantity of radicals contributing to deposition is larger than the quantity of ions contributing to etching, which is caused by an increase in the C.sub.2 /F ratio or C/F ratio in the plasma.
The microloading effect may generally be considered as phenomena occurring when the foregoing condition (c) is satisfied. Although the mechanism of these phenomena still remains to be elucidated, it can be inferred that the microloading is the phenomenon in which the etch rate is gradually reduced due to a progressive increase in the quantity of radicals supplied into the hole to form a deposit so that the functions .GAMMA.i, .GAMMA.d are expressed as continuous functions, while the etch stop is the phenomenon in which the etch rate is abruptly reduced to zero so that the functions .GAMMA.i .GAMMA.d are expressed as discontinuous functions.